This invention relates generally to imaging methods and apparatus, and more particularly to methods and apparatus for noise reduction in images. The inventive methods and apparatus described herein are particularly useful for computed tomographic (CT) imaging, and more particularly, CT perfusion imaging, but can be used with other types of images that are not necessarily CT images or medical images.
CT perfusion is an imaging technique wherein an intravenously injected contrast agent, such as a radio-opaque iodine chelate, is observed in images as the agent passes through the tissue. Cine CT imaging can provide a continuous image acquisition similar to a movie for viewing the contrast agent within the tissue. This technique usually requires an examination that lasts 45-50 seconds. Usually, a multislice detector about eight 5 mm continuous slices over a region of interest, such as a brain. When the data is obtained, the data looks like a movie, and the motion of the contrast agent can be seen as the agent passes through vessels. The vessels and tissue are also enhanced as a function of time. Maps can thus be produced that show quantitative measurements of blood flow, blood volume and transit time throughout the brain. These maps can be used by clinicians along with established techniques to assess a stroke or vascularization of, for example, a tumor.
Because a cine CT perfusion acquisition exam takes a relatively long time (typically 45-50 seconds), the technique is adjusted to provide the lowest possible x-ray dose. Accordingly, the image quality of a CT perfusion time series has a poorer signal-to-noise ratio (SNR) when compared to a CT contrast angiogram. The CT contrast angiogram is acquired using a helical acquisition. Thus, each slice acquisition is a type of snapshot, but the CT angio scan may take several seconds. The snapshot can be acquired rapidly without a lengthy x-ray dose, and thus can have a higher SNR than a CT perfusion time series image.
In magnetic resonance imaging, elliptic centric imaging acquires data in frequency space (which can be an image frequency space, for example, a k-space) directly. Every view that is acquired is a coordinate or line in frequency space, for example, starting at the y-axis in the center, or the origin. Elliptic centric acquisition is acquired over time, one point after another, for example, in successive points parallel to an axis or in a spiral from the origin when the contrast agent in the body is in the arterial phase. An acquisition is typically 15-20 seconds long. At about 5 seconds, the contrast bolus has already passed through the tissue and is already in the veins. The veins are relatively larger structures than other structures in the region of interest. The region of interest also has structures that have higher spatial frequencies. If it is known that there are no signals coming from large venous structures, the signals would not contribute to an elliptic centric image because the venous structures have most of the frequencies in the center of frequency space, which is filtered out. Thus, a relatively lengthy (15-20 second) image can be acquired with contrast injection. Because of the manner in which frequency space is acquired over time, this image looks like an artery image without veins, which are not enhanced. This technique can be used to obtain a good arteriogram free of venous contamination because of the manner in which frequency space is sampled relative to the timing of the contrast.